The present invention relates to a system for controlling the hydraulic pressure of an automatic transmission including a lockup control valve.
Typically, a hydraulic circuit for a V-belt type continuously variable transmission (CVT) comprises a lockup control valve for engaging a lockup clutch of a torque converter for direct coupling between an engine and a CVT. The lockup control valve bifurcates the hydraulic pressure drained from a clutch regulating valve into two pressures of the apply side and release side of the torque converter in accordance with a signal output from a lockup solenoid. By controlling a differential pressure between the apply-side and release-side pressures, the lockup control valve carries out engagement and disengagement of the lockup clutch.
The lockup solenoid includes a linear solenoid valve which changes a constant pilot pressure supplied from a pilot valve in accordance with a control signal output from a CVT control unit to provide it, as a signal pressure, to the lockup control valve. The linear solenoid valve has the advantage of higher responsivity than a duty control valve.
The lockup solenoid receives the pilot pressure through an input port to provide it, as a lockup-solenoid source pressure, through an output port. The lockup solenoid changes the volume defined between a valve element arranged at an end of a plunger and a bearing surface formed on a valve chest by a balance between a biasing force of the solenoid and the pilot pressure, thus changing the pilot pressure to a desired pressure.
With the above hydraulic circuit, an indicated value of the differential pressure of the lockup solenoid after engagement of the lockup clutch is set at a maximum value at all times to maximize an engagement capacity of the torque converter. Thus, a spool of the lockup control valve is in the fully biased state at all times to increase the load rigidity of a particular hydraulic passage as viewed from the lockup solenoid.
Thus, during high engine rotation having increased surging of the line pressure as source pressure of the pilot pressure, surging of the pilot pressure as source pressure of the lockup solenoid is increased, resulting in possible deterioration in the pressure-control stability of the lockup solenoid.
Further, if the pressure-control stability of the lockup solenoid is deteriorated, an indicated value of the differential pressure varies to reduce the engagement capacity of the lockup clutch, having engagement of the lockup clutch released, leading to possible degradation in fuel consumption.
A solution for absorbing surging of the pilot pressure lies in interposing an accumulator between the pilot valve and the lockup solenoid. However, this solution involves an increase in the number of parts, leading to increase in manufacturing cost and degradation in the vehicle mountability.
Another solution lies in setting the source pressure of the lockup control valve, i.e. the discharge capacity of an oil pump, at a fully high value. However, this solution involves provision of the source pressure of the lockup control valve greater than the differential pressure required for a maximum engagement capacity of the lockup clutch, increasing a load to the engine for driving the oil pump, causing degradation of fuel consumption.